It is of great importance to our health and safety to know the condition and habitability of our environment. In particular, much effort and expense has been expended in the past to control and detect pollutants in air. For example, in residential housing units including homes, office buildings, or buildings that have laboratories or where dangerous chemicals are stored, the need to monitor the air for smoke or contamination is very great. It is also necessary that air sensing be accurate and rapid which is a particular challenge in multiple spaces such as individual rooms in large office or apartment buildings.
Early attempts to address this need have produced systems capable of detecting poor ventilation in buildings by simple means, usually indicating excess CO.sub.2 levels or VOC's (Volatile Organic Compounds), excess particulates, or other dangerous gaseous materials. However, these early continuous monitoring systems were often expensive, difficult to install, and not known to be accurate or reliable due to the nature of the individual sensors themselves or the calibration required of these sensors.
Systems for sensing air quality are shown in U.S. Pat. Nos. 3,683,352; 3,781,092; 3,805,066; 4,027,153; 4,403,806; 4,516,858; 4,641,025; and 4,820,916, United Kingdom patent application GB 2,215,038, and German Patent DE 3409-618-A1.
U.S. Pat. No. 3,683,352 discloses an alarm system for sensing smoke and intruders. The '352 patent teaches the use of a single light beam source for transmitting light through remote light-to-electric transducers for detecting smoke and intruders passing through the light beams between the transducers. Detection of an intruder or smoke is indicated by an amplitude or polarization modulation of the light signal at a characteristic frequency.
U.S. Pat. No. 3,781,092 discloses a monitoring system having a plurality of independent optical fiber paths, transducers, optical shutters, and modulators in combination with a common laser light source. Light is emitted by the common source and split among the various fiber light paths present in the system. The light travels through the fiber light path to the transducer which encodes information onto the light by means of an optical switch.
U.S. Pat. No. 3,805,066 discloses a smoke detecting device using a plurality of optical fibers arranged in a series alignment leaving gaps therebetween. A light signal is transmitted through the fiber for detection of smoke by a photo-electric transducer located at the terminal end of the series alignment.
U.S. Pat. No. 4,027,153 discloses a fiber network having a passive optical coupler for the transmission of data between addressable subscriber stations present in the system. Each station has a transmitter and receiver and a specific address code for identification. Information from the stations is sampled cyclically by a common addressing unit also connected to the stations via the passive optical coupler.
U.S. Pat. No. 4,403,806, discloses a visibility measurement apparatus having a central unit for controlling an emitted light signal and a plurality of transmitter and receiver units for measuring the attenuation of the emitted light signal. The centralized unit emits the light signal which is transmitted and received by the transmitter and receiver units and returned to the central unit for evaluation.
U.S. Pat. No. 4,516,858 discloses a multiple site laser-excited pollution monitoring system having a central laser source and a plurality of optical fibers. The laser emits a light signal that is deflected in timed sequence to a plurality of remote laser-excited photo-acoustic detector heads for detecting vapor by Raman scattering, fluorescence, absorption, and photoionization. The detector heads transmit a detection signal by electric wires to a signal processor and display unit that are also located at a central location.
U.S. Pat. No. 4,641,025 discloses a system for determining the position of the boundary between substances having different refractive indices. The '025 patent discloses a plurality of optical sensors, a common pulse source, and an interface responsive to the source pulse and for producing a response pulse that is delivered to common receiver. The receiver measures the duration of the response pulse having a time duration that is proportional to the number of sensors in the system. The position of the boundary is determined by calculating the time duration in response to a fixed and known number of sensors.
U.S. Pat. No. 4,820,916 discloses an optically powered sensor system having a plurality of sensors connected to an optical bus for communication with a system controller. Optical energy is transmitted on the bus and distributed to the sensors system-wide. The sensors have a photodiode array for sensing a measurable parameter and providing an optical pulse signal as a function of the measured parameter. The sensors include a transducer and a pulse encoder for producing a series of short duration pulses to drive an optical source for transmitting corresponding optical pulses to the system controller. The patent provides for a multi-sensor configuration by providing sensor-specific time delay prior to the transmission of the return pulse from the sensor to the controller. The time delay provides a predetermined time window for each sensor allowing sensor discrimination.
United Kingdom patent application GB 2,215,038 discloses an optical sensing system including a central light source emitting broadband light over a plurality of light paths that are terminated in a common Fabry-Perot cavity filter having scanning and detector means for scanning a narrow bandwidth of the broadband light.
German Patent DE 3409-618-A1 discloses a fiber optic measurement system having a plurality of optical fibers connected to a plurality of optical sensors and light sources having different emission spectra selected according to the desired absorption and transmission characteristics of the sensors.
In addition to installed systems, other approaches for sensing the air quality in the past have included hand-held sensing instruments. However, these are expensive and awkward devices to use, particularly when long term multi-room monitoring is desired. Another approach used in the past is individual OEM type sensor instruments connected into some type of data gathering and or control system. However, these systems are also very costly if many rooms must be monitored simultaneously, since costly sensors are required in each room that is monitored. In addition, cost and complexity dramatically increase when more than one gas is to be sensed and monitored. Furthermore, operating costs of these systems are also high due to the large amount of field work required to continually recalibrate the large volume of sensors that are employed.
Recently, many types of new sensors have been developed or proposed that use optical techniques with light from lasers or other light sources. In many cases the light emitted from these sources can be transported and guided by the use of fiber optic cable made of plastic, glass or other compounds. This allows the emitter and/or detector to be remotely located from the area to be sensed. It also potentially allows the use of techniques to multiplex the use of one set of gas or particle light emitters and detectors over many measurement sites or locations. Current multiplexing approaches have included, for example, Wavelength Division Multiplexing where many distinct light signals each of a different wavelength are created and sent in to a fiber system with multiple sensors. Each sensor can respond to a different wavelength signal. These modified signals are then demultiplexed at the common detector location and individually sensed.
In another example, Time Division Multiplexing (TDM) is used to send a very short light pulse into a multiple fiber (1XN) beam splitter or coupler which creates multiple copies of the pulse on multiple fibers connected to the coupler. After passing through a sensor located on each one of these multiple fibers, all the modified pulses are recombined by another multiple fiber (1XN) coupler back onto one return fiber. As long as the path lengths of the multiple fiber-sensor paths are different, the result will be a pulse train of individually modified pulses on the single return fiber. By using time and path length the affected pulses can be matched to the appropriate sensor to detect and determine a particular condition or substance at the sensor's location.
These two multiplexing approaches are complex, expensive, and are not general purpose due to the wavelength or time limitations of the technique. Consequently they are not readily adapted to changing environmental sensing requirements in a building.
To avoid these types of limitations and complexity other systems have been developed that use optical switches to switch one of many fibers coming from a remote location to a common emitter or detector. These approaches use an optical switch which can switch a light beam from one fiber to another one of several other fibers with minimal loss and affect on the transmitted light. These approaches specifically locate the multiplexing switch near the emitter and or detector and use a multitude of fiber cables leading from the central location to the sensed locations. This approach although simpler and more flexible than the previous approaches suffers from the need to locate a vast amount of fiber cables throughout the building. If a new location is to be added it requires the installation of another fiber cable between the central location and the new sensed location.
The present invention addresses and solves many of the above-mentioned problems associated with prior art systems.